A British official says the U.K. Parliament would have a role in blocking EU legislation that it disagrees with under draft rules for a new relationship between Britain and the European Union.

The plan is being negotiated in advance of a promised referendum on British membership that Prime Minister David Cameron has promised to hold by the end of 2017.

The details of the proposal have not yet been made public. European Council President Donald Tusk is expected to outline the terms later Tuesday.

The British official made the comments in a statement released only on condition of anonymity because the EU had not yet made its official announcement.

The official said that under the plan, if 55 percent of European parliaments disagree with EU legislation it could be blocked.